1. Field of the Invention
The present invention relates to optical communications systems and, in particular, to a bit rate-independent optical receiver for use in optical communications systems.
2. Description of the Related Art
In general, optical transmission systems are provided with various communications protocols, i.e., Fiber-Distributed Data Interface (FDDI), Enterprise Systems Connectivity (ESCON), Fiber Channel, Gigabit Ethernet and/or Asynchronous Transfer Mode (ATM). These protocols operate at different bit rates, including 125 Mb/s, 155.520 Mb/s, 200 Mb/s, 622.080 Mb/s, 1062.500 Mb/s, 1.25 Gb/s or 2488.320 Gb/s, etc. As a predetermined bit rate of the optical signal is normally applied to an optical communications system with a selected specific protocol/bit rate, an optical receiver for use in the optical communications system must meet the predetermined bit rate. The function of the optical receiver is to convert an input optical signal into an electrical signal and then recover from the electrical signal the original data without noise.
FIG. 1 shows a schematic block diagram of an optical receiver according to a prior art and includes an opto-electric converter 110, a low-noise amplifier 120, a limiting amplifier 130, and a clock/data recovery circuit 140. In use, the opto-electric converter 110 serves to convert an input-optical signal into an electrical signal. The low-noise amplifier 120 serves to eliminate noise from the electrical signal and amplify the electrical signal by a predetermined amplifying factor, while the limiting amplifier 130 serves to re-amplify the amplified electrical signal of the low-noise amplifier 120 within the limit of a predetermined voltage level. The clock/data recovery circuit 140 includes a phase-locked-loop (PLL) circuit with a reference-clock generator for generating the reference clock of a single frequency corresponding to a particular bit rate. The clock/data recovery circuit 140 serves to reshape the re-amplified electrical signal on the basis of the reference clock of the reference clock generator.
However, the aforementioned optical receiver usually does not adapt easily to any changes in the transmission format and its bit rate (so-called “transparency”). Thus, it has drawbacks in that it is not applicable to optical communications systems in which more than one transmission format is intended for use, and a frequent change in the transmission format is required.
Furthermore, such non-transparency makes it difficult for the optical communications system to perform a real-time monitoring operation in relation to any deteriorative accumulation of the optical signals, such as amplification noise occurring in between network sections, non-linearity of the optical signals, dispersion of color, etc. Therefore, it causes a considerable limitation in some practical applications of optical networks, i.e., optical line dividers.